This invention relates to tubular substrates having electrical conductors on an interior surface that are electrically connected through the tube wall to contacts on the tube exterior. Preferably, the conductors are concentric rings for a drift tube of an Ion Mobility Mass Spectrometer (IMS). This invention further relates to methods for producing such tubular substrates from flexible polymeric printed wiring board materials, ceramic materials and material compositions of glass and ceramic, commonly known as Low Temperature Co-Fired Ceramics (LTCC).
In the context of the present invention, LTCC is generally defined as a family of glass-ceramic dielectric substrate materials that are flexible and formable in the ‘green’ unfired state, and become rigid upon firing at temperatures below 1000° C. Such dielectric materials are widely used in the art and are supplied as flexible green sheets or tapes. Conductors, resistors, via fills, inductive and capacitive structures are incorporated in or printed on the LTCC dielectrics using compatible inks or pastes by processes well known in the art. These materials and recommended processing methods are well known and available commercially from Electro-Science Laboratories, INC, King of Prussia, Pa., the DuPont Company, Wilmington, Del. and Ferro Corporation of Cleveland, Ohio, among others.
An IMS is an analytical instrument that performs vapor phase compound speciation based on ion mobility in an atmospheric environment. Uses for an IMS include analyzing air samples for the presence of harmful substances in the form of toxic vapors, components of explosive materials, trace constituents of drugs, sampling of biological systems such as biogenic amines or fatty acid methyl esters, or other analytical chemistry applications. In a typical configuration, an IMS consists of a drift tube with drift gas inlet at one end, connected to an ionizing chamber at the opposed end, the ionizing chamber being provided with a sample gas inlet port. Gating grid electrodes are located between the drift tube and the ionizing region. An exhaust outlet for gases is provided for near the ionizing chamber and away from the gating electrodes. The drift gas, typically dry air, nitrogen or other inert gas, flows along the drift tube, past the sample inlet port, through the ionizing region and exits via the exhaust outlet. The sample to be analyzed is introduced along with a carrier gas, typically dry air, nitrogen or other inert gas, through the sample inlet port into the ionizing region where isotopic radiation, corona discharge sources, ultraviolet radiation or other known ionization techniques, ionize the constituents of the carrier and sample gases either by primary ionization of the analyte or secondary ionization by chemical reaction with a supplied dopant. Ions created in the ionizing region are attracted towards the entrance of the drift tube by the presence of an electric field. By applying a voltage pulse on gating grid electrodes separating the ionizing region from the drift tube, a controlled flow of ions is allowed into the drift tube, along which a generally linear potential gradient has been established. Ions within the drift tube are accelerated by the potential gradient and migrate against the drift gas flow, towards the ion detector. The rate or drift speed (Vd) at which ions migrate through the drift tube is controlled by their mobility (K) and the magnitude of the electric field (E) according to the following relationship Vd=KE. The ion's mobility is a generalized parameter that includes effects such as charge, collision cross-section, reduced mass of the ion-neutral collision pair, ion polarizability, temperature, and other variables. Chemical speciation is achieved by electronic circuits which monitor the ions collected on the detector, as a function of time passed since application of the voltage pulse to the gating electrodes. The resulting waveforms representing the number of ions collected versus time are well known to be indicative of the chemical species and their relative quantities, present within the gas sample being analyzed.
The construction of the drift tube, field and gating electrodes, ionizing region, gas inlet and exhaust port attachments, has significant impact in determining the size, cost, and performance of the IMS as a practical instrument. To achieve an optimized IMS, it is desired that the drift tube assembly be relatively small in size, airtight, constructed of non-contaminating materials, and of low cost to produce.
The following references teach several approaches for construction of the drift tube and it's associated components.
U.S. Pat. No. 4,390,784 to Browning et al discloses an ion accelerator for an ion mobility detector cell that is comprised of a ceramic tube coated inside with a thick film resistor composition across which a voltage potential difference is impressed to provide an ion accelerating electrical field gradient within the tube. One such tube is used to define the mobility detector reactant region and a second similar tube is used to define the drift region. Gating grid electrodes are provided by wire screens or mesh, contained in a separate mechanical assembly of many parts, placed inside the ceramic tube. This approach provides a functional system albeit one that is not amenable to miniaturization or low cost of manufacture.
U.S. Pat. No. 5,280,175 to Karl discloses a drift tube produced by stacking metal ring electrodes separated by insulating rings. The stack-up of conducting and insulating rings would be sealed with conventional o-rings, or brazed into an airtight assembly. To provide a voltage divider for creating a potential gradient along the drift tube, the ring electrodes would be interconnected with deposited or discrete resistor elements. The use of brazing to join the piece parts would produce a relatively inert, non-contaminating drift tube but requires a complicated assembly process.
U.S. Pat. No. 5,965,882 discloses a drift chamber created in the space between two printed wiring boards, separated by a Teflon™ spacer. Necessary electrodes are provided on the printed wiring boards and sealing of the drift chamber is accomplished by mechanical compression of the Teflon™ spacer. Systems constructed of such organic materials can be troubled with outgassing of contaminants from these materials, and may require extended flushing of the system to achieve optimum sensitivity.
U.S. Pat. No. 6,051,832 discloses a volume enclosure built up upon the surface of a printed wiring board by soldering a multiple of stamped metal assemblies onto it. A larger metal stamping is used to form the drift tube enclosure. Additional stampings are located within this volume and function as the electrode elements, interconnected by surface mounted discrete resistors. The joints within this structure are soldered to create an airtight enclosure. This system provides for integrating the electronics near the drift tube, but again requires a complex assembly process.
The applicants have described yet another approach to producing the drift tube assembly in the publication: “Applying New LTCC/LIGA Construction Techniques in Realizing a Miniature Ion Mobility Spectrometer”, presented at the conference on Packaging of MEMS and Related Micro Integrated Nano Systems, sponsored by the International Microelectronics and Packaging Society, Sep. 7, 2002, Denver, Colo. This publication discloses an approach to miniaturizing an IMS by constructing a drift tube of alternately stacked sapphire insulators each having a centrally located hole, with thin metal plates each having a centrally located hole or screen feature to function as an electrode. In this approach, the many alternating layers of insulator and metal electrode plates are joined with adhesives or metallurgically bonded through brazing or soldering, to produce an airtight assembly. Machined blocks of ceramic or similar material are bonded at each end of the drift tube to provide inlet and exhaust gas ports and provide for the ionizing and reaction regions. Manufacture of such a device proved successful, but was labor intensive and prone to leakage, as is typical for a structure with a large number of joints, each required to be airtight. A need exists for a simpler method for producing a structure usable as the drift region of an IMS, that preferably would entail non-contaminating materials and minimize the number of airtight joints required.
It is desired to provide a method of forming tubular substrates beginning with readily available planar sheets of ceramic, glass-ceramic or suitable polymeric materials. The following references teach several approaches for forming three-dimensional structures in ceramic materials.
U.S. Pat. No. 3,755,891 to Muckelroy et al discloses a method for producing three-dimensional circuit modules utilizing thick-film manufacturing processes applied to the inner and outer surfaces of a provided substrate in the form of a ceramic cylinder. Circuit networks are printed on the inside and outside of the cylindrical substrate, utilizing screen printers having similarly shaped cylindrical screens. After firing of the circuit networks, interconnection is effected by conductive clips extending from conductors on the inner surface of the cylinder to conductors on the outer surface. While a method as taught by Muckelroy et al might provide a basis for developing a drift chamber, the requirement to print networks on the interior of a tube is difficult to manufacture, and necessarily restricts the process to tubular structures with a large inner diameter, to accommodate the screen printing means.
U.S. Pat. No. 4,475,967 to Kanai et al teaches a method for producing a ceramic capacitor by rolling a sheet of green ceramic material about a core. A conductor pattern of alternating electrodes, separated by insulating gaps, is printed on one surface of the green ceramic sheet. By eliminating the necessity to maintain accurate alignment of two green ceramic sheets, problems attributed to miss-alignment of the sheets could be avoided, such as increasing the scatter of capacitance values produced or short-circuits developing between the electrode patterns. Kanai et al does not provide a method for interconnecting inner and outer electrodes as these ‘short-circuits’ would be deleterious to producing a capacitor, and is cited as a disadvantageous by-product of processes requiring accurate alignment of two green ceramic sheets.
U.S. Pat. No. 5,028,473 to Vitriol et al discloses forming an electrical circuit pattern on a glass-ceramic thermally fusible tape. The tape is heated to a temperature at which it becomes temporarily plastic, and is then bent into a desired non-planar shape. A multi-layer structure can be provided by laminating together plural layers of LTCC tape with respective circuit patterns formed thereon, and plastically bending the laminated structure into the non-planar shape during a heating step. Interconnection of circuit patterns on different tape layers is by means of electrical vias formed through a tape layer. By this method, a circuit structure including a desired edge connector can be formed into a non-planar shape. Examples of bending glass-ceramic tape by heating the tape and shaping or pressing the softened tape about a mold form are provided. While this method can produce a shaped substrate, this method requires handling of glass-ceramic substrates heated to their softening point. Having to form the substrate while in the heated state unnecessarily complicates the process and exposes external conductor and resistor networks to potential damage during handling.
U.S. Pat. No. 6,527,890 to Briscoe et al teaches a method for producing a textured channel in a plurality of green-sheet layers, which are then sintered together to form a substantially monolithic structure. Such a channel, filled with a porous phase for differentially adsorbing chemical components, can be used as the column for separation of gas species by gas chromatography. While this method can produce a channel in a planar substrate, ionizing of species and control of ion flow through appropriate application of an electric field gradient is not required for separation of gas species in Briscoe's disclosed method, and thus there is no provision for producing such.
Each of the above references describes an approach having unique qualities, but no reference on it's own satisfies all of the required characteristics. There remains a need in the art for a more advanced and efficient method of producing a tubular structure with the necessary electrode elements, as can be utilized as a drift tube in an IMS. It is therefore an object of this invention to provide a drift tube that is relatively small in size, airtight, constructed of non-contaminating materials, and of low cost to produce.